Question

a student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens . he noted his observations as :
it is in the image

Answer 1

you can see, case (iii) ; here both the object and the image distances are 30cm .but we know this is possible only case when object kept at 2f ,so, 2f = 30cm => f = 15cm

(b) use lens formula for finding incorrect observations.
1/v - 1/u = 1/f
see last observation (v)
u = -15
f = +15
then, 1/v = 1/15 - 1/15 = 0 = 1/∞ => v = ∞
but v = 70cm is given hence, observation (v) is wrong.

(c) obviously case 3 , in this case size of image is equal to size of object due to image distance distance is equal to object distance from pole.

Answer 2

Answer of above question is following